The invention relates generally to a method for recovery of tin, tin alloys or lead alloys, such as solder, from printed circuit boards.
With the advent of more and more electronic hardware, there is more and more electronic scrap produced, including scrap printed circuit boards. Furthermore, there is a growing need for more environmentally friendly methods of recycling scrap printed circuit boards. Many current commercial processes focus only on recovering some of the component parts of printed circuit boards. For example, printed circuit boards are heated in furnaces to pyrolyze the organic compounds and melt the metals. The plastics and the electronic components are destroyed producing potentially harmful fumes and a remaining complex mixture of metals that then needs to be separated. An alternative process comprises the selective removal of the higher value components, such as gold edge connectors and microprocessor chips, and then scrapping the rest of the printed circuit board to, for instance, landfill.
It is more likely than not that legislation will be planned and enacted, particularly in the European Community, that will require manufacturers of electronic equipment to take back equipment at the end of its useful life for recycling. There is, therefore, a need for a method for treating printed circuit boards that can recover the majority of the cornconent parts of scrap printed circuit boards without the production of undesirable waste products.
During the manufacture of printed circuit boards, solder is applied at various stages for the purpose of bonding components to the exposed areas of metal using a wave soldering technique or using a solder paste. Furthermore, during the formation of lines and pads on printed circuit boards, a layer of tin or a tin-lead alloy, such as solder, may be deposited by circuit boards, a layer of tin or a tin-lead alloy, such as solder, may be deposited by electrodeposition, immersion plating or other processes onto selected areas of copper surfaces of circuit boards to define conductive tracks, pads, through holes, etc., and to act as an etch resist during subsequent etching array of the copper surfaces. Thus, in a method for recycling of scrap printed circuit boards the selective dissolution of the tin or tin-lead alloy used in the manufacture of the printed circuit boards from the copper substrate surface, without dissolving the copper, is an important first step.
European Patent No. EP-0508960 discloses a method of producing metallic lead from a material containing lead by dissolving the lead contained in the material and an electrolysis step for the cathodic deposition of the dissolved lead. The dissolving step is carried out with the use of an acid electrolyte in the presence of a redox couple. The method is used, in particular, for the production of lead from the active material of spent accumulators. Although this process discloses the dissolution of lead, a process for the selective dissolution of tin or tin-lead alloys is still desired.
A new selective dissolution process has now been developed in which the Plastics associated with electronic components are not attacked, the electronic components are not affected by the oxidant solution used in the process and the identification coding on the electronic components is unaffected. Thus, the components can be recovered for use. Furthermore, the oxidant solution, which is used to dissolve the tin or tin-lead alloy, is regenerated and recycled to the method, thereby avoiding the production of undesirable or unwanted waste products.
Embodiments of the invention provide a method for selective dissolution of tin and/or lead or tin-containing alloys from printed circuit boards without substantial dissolution of copper or precious metals and without attack on plastics associated with electronic components mounted to printed circuit boards, the method comprising steps of:
(i) providing a solution comprising Ti(IV) and an acid which forms stable and soluble salt of Ti(III), Ti(IV), Sn(II) and Pb (II);
(ii) contacting a printed circuit board with the solution under anaerobic conditions for a time sufficient to effect dissolution of substantially all of the Sn and/or Pb- or Sn-containing alloy therefrom, as Sn(II) and/or Pb(II);
(iii) subjecting the solution from step (ii) to electrolytic reduction in order to recover substantially all of the Sn(II) and/or Pb(II) species contained therein as Sn and/or Pb; and
(iv) oxidizing the solution from step (iii) to regenerate the oxidant metal species and recycling the regenerated solution to step, (i) of the process.
By the term xe2x80x9csoluble saltxe2x80x9d as used herein is meant that the salt has a solubility of at least 1 gram per liter in the solution at the temperature at which the process is carried out.
For use in step (ii) of the method according to the invention, the oxidizing solution will preferably oxidize tin and lead metals, but will not dissolve copper. For this to be achieved the solution redox potential should be in the range of from +0.342 volts to ÷0.126 volts (standard hydrogen electrode), in terms of standard electrode potential, i.e. the interval between the anodic dissolution of copper and tin/lead.
The preferred solution for use in step (ii) of the method comprises an acid selected from the group consisting of fluoroboric acid, fluorosilicic acid, hexafluoro-phosphoric acid, hexafluoroantimonic acid and an alkylsulphonic acid. A solution comprising Ti(IV) and fluoroboric acid is particularly preferred. This solution may be prepared by dissolving Ti metal in fluoroboric acid with a sparge of oxygen gas or air. This solution has an unexpected selectivity for the dissolution of Sn and Pb over other metals, particularly copper.
In one embodiment of the method of the invention, other oxidant species/acids as defined in step (i) may also be used provided that, in use, the oxidant ions in solution have an oxidation potential which can selectively dissolve tin or a tin-lead alloy from the surface of the printed circuit board, but does not oxidize copper metal. Step (ii) of the method is carried out anaerobically, for example, by sparging the leach solution with an inert gas, such as nitrogen, during the leaching step.
The solution used in step (ii) of the method preferably comprises the oxidant metal species at a concentration generally in the range of from 0.01 M to 1.0 M metal ions, preferably 0.5 M to 0.9 M metal ion, and the acid at a concentration in the range of 10% to 100% by weight acid.
The temperature at which the solution used in step (ii) is contacted with the substrate surface is generally from 0xc2x0 C. to 60xc2x0 C., and preferably 20xc2x0 C. to 30xc2x0 C.
The method according to the invention may be used to dissolve tin or tin-lead alloys from printed circuit boards. Alloys which can be dissolved generally comprise from 5 to 99.5% by weight Sn, up to 95% by weight of Pb, and optionally up to 5% by weight of one or more of Ag, Bi, In, Zn, Cu or Sb. Solders commonly used in printed circuit board manufacture comprise 63% Sn, 37% Pb by weight, or 62% Sn, 36% Pb, 2% Ag by weight. Aluminium and any iron present in the printed circuit boards will dissolve in the acid leach and reduce the Ti(IV) oxidant to Ti(III), thus leaving less Ti(IV) available to react with the solder.
Accordingly, it is advantageous to remove these metals from the scrap printed circuit board prior to treatment with the leach solution. For example, physical separation methods may be used, such as by magnetic means for iron and using an eddy current separator for aluminium.
In step (iii) of the method of the invention, the electrolytic reduction of the solution containing the dissolved species is preferably carried out in a divided cell with inert electrodes, such as graphite anodes and stainless steel cathodes, and anolyte and catholyte compartments separated by a membrane, such as microporous or ion exchange material. The solution containing the dissolved species comprises the catholyte and the reaction at the cathode is the reduction of Sn(II) to Sn or the reduction of Sn(II) and Pb(II) to Sn and Pb, which are deposited on the cathode. When Pb and Sn are present together, for example as a solder alloy, they are then deposited together as an alloy, which can be re-used. The anolyte may comprise the same solution, or any other electrolyte of choice.
It is generally preferred for the catholyte solution to be isolated from the air since any oxygen present will oxidize Ti(III) in solution to Ti(IV), which in turn is reduced back to Ti(III) at the cathode. Furthermore, Sn(II) would be oxidized to Sn(IV) from which it is difficult to electrowin tin. Reduction of Ti(IV) to Ti(II) occurs at the expense of the metal plating reaction, leading to low metal yields and current efficiency.
The electrolytic reduction in step (iii) of the method of the invention is generally carried out at a current density in the range of from 50 A/m2 to 500 A/m2. The electrowinning of the tin and/or lead in step (iii) is carried out until substantially all of the metals are electrowin. The reason for this is that any Sn(II) present in the solution during step (iv) of the process would be oxidized to Sn(IV). In this form, it is very difficult to electrowin tin metal as normally the preferred cathodic reaction would be the evolution of hydrogen.
The oxidation in step (iv) of the method of the invention may be carried out by the electrolytic oxidation of the solution from step (iii). The oxidation of Ti(III) to Ti(IV) is preferably carried out in a divided cell with inert electrodes and anolyte and catholyte compartments divided by a membrane separator, such as a cation exchange membrane, for example Nafion. The oxidation of Ti(II) to Ti(IV) takes place at the anode with the solution from step (iii) forming the anolyte. The catholyte may comprise the same solution, or any other electrolyte of choice. The solution treated according to step (iv) is thus regenerated and then is recycled to step (i) of the process. Both steps can advantageously be carried out in the same cell. The leach solution loaded with Sn and Pb is first electrolyzed as the catholyte where SnPb is plated onto the cathode. The metal depleted solution can then become the anolyte where Ti(III) is oxidized to Ti(IV) at the anode after as much tin as possible has been electrowin as solder at the cathode. This is because if any tin is still present when the solution is oxidized either with air/oxygen or electrochemically, then the Sn(II) species is converted to Sn(IV) ions. In the Sn(IV) form it is difficult to convert into the metal for the reasons as discussed above. However, alternatively two divided cells can be used.
In one embodiment of the method of the invention, the oxidation may be carried out by contacting the solution with air or oxygen gas, for example, by sparging the solution with oxygen or preferably with air at high sparge rates. Alternatively oxygen gas can conveniently be generated electrolytically at the anode of the divided cell used for plating Sn/Pb. This requires no extra energy and so would be cheaper than using bottled oxygen gas.
As previously stated, the method of the invention is particularly applicable to the removal of tin, and/or alloys containing lead and/or tin, from printed circuit boards and the subsequent removal and recovery of electronic components, such as transistors, capacitors and resistors, from printed circuit boards after treatment in accordance with the method of the invention. This is an important feature and advantage of the method of the invention because the electronic components are recovered in a functional state.
In one embodiment, the method of the invention further comprises a step of contacting the desoldered, and optionally component-stripped, printed circuit board with a copper leaching solution suitable for dissolving Cu for a time sufficient to effect dissolution of substantially all of the Cu from the printed circuit board. The copper leaching solution may comprise, for example, copper (II) chloride, sodium chloride and hydrochloric acid or an ammonium salt, ammonia and, optionally, Cu(II) ions, such as, for example with ammonium sulphate/ammonia, which is most preferred. The copper leaching step will generally be carried out at a temperature in the range of from 30xc2x0 C. to 60xc2x0 C., and preferably from 45xc2x0 C. to 55xc2x0 C. In the case of Cu(II) chloride leaching optionally the Cu(II) species present in the resulting leaching solution at the end of the dissolution step may be electrolytically reduced to copper. Also, some of the Cu(I) can be oxidized anodically back to Cu(II), for further leaching, either in a divided cell or with hydrogen peroxide.
Furthermore, prior to the copper leaching step, the printed circuit board may be comminuted by one or more mechanical processing steps. This is important for multilayer printed circuit boards which contain thin layers of copper sandwiched between layers of plastic laminate. For the copper leach solution to gain access to this metal, the boards can be shredded or broken up in some way to create small pieces in which the maximum distance of any copper metal to an external edge is less than 10 mm, and preferably less than 5 mm. In one embodiment, additional process steps may be included to further recover valuable components from the scrap printed circuit boards. The components may be sorted in order according to value of component, value of contained element or toxicity of contained element, and the decoppered plastic laminate could be treated further to recover any bromine that may be present from flame retarding additives.
After the copper leaching step, the residues will still contain all of the precious metals (if present originally). The precious metals may be recovered by methods well known in the art, such as, for example, leaching with cyanide to recover gold, or leaching with chlorine, including, for example, chlorine generated electrochemically. All of the precious metals can thus be separated from the residual plastics and ceramic materials.
Illustrative embodiments of the method according to the invention are described below in further detail with reference to Examples 1-3.